Resonance tag with temperature sensor

ABSTRACT

A temperature-history sensor includes a resonance circuit composed of at least a capacitor and a coil. The temperature-history sensor has a display for indicating a predetermined set temperature of the temperature-history sensor. The capacitor has at least a thermofusion material between electrodes of the capacitor, and the melting point of the thermofusion material is in the region of the set temperature.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention provides a low-cost resonance tag, which indicatesthe temperature history and which is a new application of a wirelesstag, wherein the temperature history of a substance to be measured caneasily be checked.

2. Description of the Related Art

Japanese Patent Laid-Open No. 2004-245607 discloses atemperature-history sensor having a plurality of temperature switchesand LC resonance circuits, in which the value of capacitance element isswitched in accordance with turning on or turning off of the temperatureswitch.

The temperature-history sensor described in Japanese Patent Laid-OpenNo. 2004-245607 can keep the temperature history stepwise andirreversibly. However, a commercially available temperature switch isused. Therefore, a temperature-history sensor suitable for selecting aset temperature more easily has been required.

Japanese Patent Laid-Open No. 2004-144683 describes a tag with atemperature sensor having a resonance circuit composed of a capacitorformed by using a material which has a dielectric constant varying inaccordance with temperature changes.

However, regarding the tag with the temperature sensor described inJapanese Patent Laid-Open No. 2004-144683, the variation of thedielectric constant in accordance with the temperature change isreversible. Therefore, The history of sensed temperature change must bestored in nonvolatile memory included in the tag.

SUMMARY OF THE INVENTION

According to the present invention, a low-cost irreversibletemperature-history sensor can be provided, wherein no nonvolatilememory is required and the set temperature can easily be selected.

A temperature-history sensor according to an aspect of the presentinvention includes a resonance circuit composed of at least a capacitorand a coil, wherein the above-described temperature-history sensor has adisplay for indicating a predetermined set temperature of theabove-described temperature-history sensor, the above-describedcapacitor has at least a thermofusion material between electrodes of theabove-described capacitor, and the melting point of the above-describedthermofusion material is in the region of the above-described settemperature.

The region of the set temperature can be the range from a temperature0.5° C. lower than the above-described set temperature to the settemperature.

The above-described display can indicate the above-described settemperature by an electrical method, a magnetic method, an opticalmethod, or a method by using printing.

At least a part of the above-described thermofusion material between theabove-described capacitor electrodes can flow out due to heat so as tochange the resonance characteristic of the above-described resonancecircuit.

A temperature-history sensor according to another aspect of the presentinvention includes a resonance circuit composed of at least a capacitorand a coil, wherein the above-described temperature-history sensor has adisplay for indicating a predetermined set temperature of theabove-described temperature-history sensor and a thermofusion material,the melting point of the above-described thermofusion material is in theregion of the above-described set temperature, and at least a part ofthe thermofusion material is fused due to heat and flows between theabove-described capacitor electrodes so as to change the effectivedielectric constant of the above-described capacitor.

The above-described capacitor can include a porous dielectric betweenthe above-described capacitor electrodes.

The above-described thermofusion material can be disposed in thevicinity of the above-described capacitor electrodes.

A flow path for supplying the above-described thermofusion material canbe disposed between the above-described capacitor electrodes.

A temperature-history sensor according to another aspect of the presentinvention includes a resonance circuit composed of at least a capacitorand a coil, wherein the above-described temperature-history sensor has athermofusion material, and at least a part of the thermofusion materialis fused due to heat and flows between the above-described capacitorelectrodes so as to change the effective dielectric constant of theabove-described capacitor.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram (one capacitor and one coil) showing anexample of an equivalent circuit of a resonance tag with a temperaturesensor.

FIG. 2 is a schematic diagram (parallel connection) showing an exampleof an equivalent circuit of a resonance tag with a temperature sensor.

FIG. 3 is a schematic diagram (series connection) showing an example ofan equivalent circuit of a resonance tag with a temperature sensor.

FIGS. 4A and 4B are external views showing an example of a resonance tagwith a temperature sensor in the case where a dielectric is air.

FIGS. 5A and 5B are sectional views showing an example of a resonancetag with a temperature sensor in the case where a dielectric is air.

FIG. 6 is a diagram showing a resonance characteristic after a heattreatment of a tag with a temperature sensor in the case where adielectric is air.

FIGS. 7A and 7B are external views showing an example of a resonance tagwith a temperature sensor in the case where thermofusion materials aredisposed between electrodes in advance.

FIGS. 8A to 8D are sectional views showing an example of a resonance tagwith a temperature sensor in the case where thermofusion materials aredisposed between electrodes in advance.

FIG. 9 is a diagram showing a resonance characteristic of a tag with atemperature-history sensor in the case where thermofusion materials aredisposed between electrodes in advance.

FIGS. 10A and 10B are external views showing an example of a resonancetag with a temperature sensor in the case where a dielectric is a porousmaterial.

FIG. 11 is a sectional view showing an example of a resonance tag with atemperature sensor in the case where a dielectric is a porous material.

FIG. 12 is a diagram showing a resonance characteristic after a heattreatment of a tag with a temperature sensor in the case where adielectric is a titanium oxide film.

FIG. 13 is a diagram showing a resonance characteristic after a heattreatment of a tag with a temperature sensor in the case where adielectric is a polystyrene film.

FIGS. 14A and 14B are plan views showing an example of a tag with afunction to visually recognize a temperature.

FIGS. 15A and 15B are sectional views showing an example of a tag with afunction to visually recognize a temperature.

FIGS. 16A and 16B are external views showing an example of a resonancetag with a temperature sensor having a flow path.

FIG. 17 is a sectional view showing an example of a resonance tag with atemperature sensor having a flow path.

DESCRIPTION OF THE EMBODIMENTS

The embodiments according to the present invention will be describedbelow with reference to the drawings. Regarding the drawings referred toexplain the present invention, the same elements are indicated by thesame reference numerals, and duplicate explanations thereof will not beprovided.

FIGS. 1 to 3 are schematic diagrams showing examples of tags with atemperature sensor composed of a coil and at least one capacitor. Here,L denotes a coil, and each of C₁ to C₃ denotes a capacitor.

FIG. 1 shows an equivalent circuit in which one capacitor and one coilare connected. FIG. 2 shows an equivalent circuit in which threecapacitors and one coil are connected in parallel. FIG. 3 shows anequivalent circuit in which three capacitors and one coil are connectedin series.

In the case where the tag includes two or more capacitors, as shown inFIGS. 2 and 3, the tag can indicate the temperature history stepwise.Furthermore, the history of specific temperature can be indicatedstepwise by combining a plurality of tags, each including a capacitorhaving a capacitance different from each other. The temperature historyis measured on the basis of a change in capacitance component orinductance of the circuit. Therefore, a stepwise temperature history canbe kept in either case where the connection method of the capacitor isthe series connection or the parallel connection.

The resonance tags shown in FIGS. 1 to 3 will be described in detailwith reference to the following embodiments.

First Embodiment

The present embodiment relates to a temperature-history sensor in whichat least a part of a thermofusion material is fused due to heat andflows between capacitor electrodes so as to change the effectivedielectric constant of the capacitor.

FIGS. 4A and 4B show an example of a tag with a temperature sensor ofthe present embodiment. FIGS. 4A and 4B are plan views of the tag. FIG.4A is a front-side view and FIG. 4B is a back-side view. FIG. 5A is asectional view of a section cut with a VA-VA plane shown in FIG. 4A, andFIG. 5B is a sectional view of a section cut with a VB-VB plane shown inFIG. 4A.

The tag with a temperature sensor of the present embodiment includes aninsulating substrate 1, a capacitor lower electrode pattern 2 disposedon the insulating substrate 1, and an electroconductive wiring pattern 3corresponding to a coil antenna portion, a pad portion 4, which isconnected to one end of an upper electrode and which has a function ofsupporting the upper electrode, and a pad portion 5, which is to beconnected to an electroconductive member in a via hole, at the two endsof the electroconductive wiring pattern 3. The electrode pattern 2 iselectrically connected to a pad portion 6 at one end of a wiring(back-side wiring) disposed on the back-side surface of the insulatingsubstrate 1 through the electroconductive member in the via hole 15. Apad portion 7 at the other end of the back-side wiring is electricallyconnected to the pad portion 5 through the electroconductive member inthe via hole 15. That is, the electrode pattern 2 is electricallyconnected to the pad portion 5.

A dielectric material 8 is disposed on the lower electrode pattern 2 ofthe capacitor. Furthermore, an upper electrode 9 of the capacitor isdisposed on the dielectric material 8. In the present embodiment, air isused as the dielectric material 8. Therefore, spacers 14 are disposed tokeep the gap between the capacitor electrodes. For the dielectricmaterial 8, any material can be used insofar as the material has a spaceinto which a melt of a thermofusion material, as described later, canflow. Examples of dielectric materials include porous solid dielectricmaterials. In the case where a material, e.g., a solid, keeping aconstant shape is used as the dielectric material, the spacer 14 neednot be disposed.

The tag of the present embodiment includes the dielectric materialbetween the electrodes, and further includes opening portions 10 toexpose the dielectric material to the outside.

The positions of the opening portions can be appropriately determined inrelation to the positions at which the thermofusion materials, asdescribed later, are disposed. For example, in the case where thethermofusion material is disposed on the upper surface of the upperelectrode 9, the opening portion may be disposed in such a way as topenetrate the upper electrode.

Thermofusion materials 11, 12, and 13 having different melting pointsare disposed in the vicinity of the opening portions 10. In Claims andspecifications of the present invention, the term “vicinity” refers to adistance of 1 mm or less. When the temperature of the environment, inwhich the tag is placed, reaches the predetermined set temperatures ofthe above-described temperature-history sensor, the respectivethermofusion materials 11 to 13 are fused in increasing order of meltingpoint and are converted to melts. In the present example, these meltsflow into gaps sandwiched by the spacers, the upper electrode, and thelower electrode. Consequently, the effective dielectric constant of adielectric layer is changed, and the resonance characteristic (thecapacitance or the inductance of the resonance circuit) of the tag ischanged. In the present embodiment, the air is used as the dielectricmaterial. In the case where, for example, a solid dielectric havingpores is used as the dielectric material, the melt flows into the poresand, thereby, the resonance characteristic of the tag is changed.

In the present embodiment, the dielectric material 8 is required to havea space, into which the melt flows. Strictly, in the case where the airitself is used as the dielectric material, at least a part of the air isreplaced with the melt. In the case where the dielectric material is asolid dielectric having a space, e.g., pores, at least a part of the airin the pores or the like is replaced with the melt. Even in the lattercase, it is possible to assume that the air is also a part of thedielectric material. In either case, the space is necessary. Therefore,the dielectric material can be a solid material (solid dielectricmaterial) having pores or be the air. Examples of such solid materialsinclude ceramic materials, e.g., barium titanate, titanium oxide, andaluminum oxide, and resin materials, e.g., polystyrenes, polymethylmethacrylates, polyimides, polypropylenes, ABS resins, and polyphenylsulfide resins.

For the insulating substrate 1, a resin substrate, a glass substrate,and the like can be used. Among them, the resin substrate can below-cost and light-weight. Examples of materials for the resin substrateinclude polyimide resins, epoxy resins, glass fiber-reinforced epoxyresins, polyethylene terephthalate resins, polyethylene naphthalateresins, phenol resins, and acrylic resins, which are industrially usedas electronic component substrates in general. Most of all,heat-resistant resins, e.g., polyimides, can be used in consideration ofthe probability of an occurrence of unexpected high-temperature state,from the viewpoint of the function as a temperature sensor.

The materials for the electroconductive wiring pattern 3, the lowerelectrode pattern 2, the upper electrode 9, the pad portions 4 to 7, theelectroconductive member disposed in the via hole 15 can be materialsexhibiting the electrical conductivity. For example, noble metals, e.g.,gold, silver, and copper, and electroconductive polymers represented bypolyanilines, polythiophenes, and polypyrroles can be used. Theseelectroconductive members can be formed by, for example, a method byusing plating or a method by using printing, e.g., a nanoimprintingmethod, a screen printing method, or the like.

The thermofusion materials 11 to 13 are materials having melting pointsin the region of the predetermined set temperatures of theabove-described temperature-history sensor. Here, the region of the settemperature can be the range from a temperature 0.5° C. lower than theset temperature to the set temperature. Put another way, the meltingpoint can be within the range of the set temperature −0.5° C. and theset temperature. For example, in the case where the set temperatures ofthe temperature-history sensor are A° C., B° C., and C° C., the tagincludes a thermofusion material having the melting point of (A° C.-0.5°C.) or higher, and A° C. or lower, a thermofusion material having themelting point of (B° C.-0.5° C.) or higher, and B° C. or lower, and athermofusion material having the melting point of (C° C.-0.5° C.) orhigher, and C° C. or lower.

In the case where the temperature-history sensor has oneset-temperature, the information whether the temperature-history sensorhas experienced the temperature higher than or equal to the settemperature or not can be obtained from the temperature-history sensor.For example, in the case where the set temperature is A° C. and themelting point of the thermofusion material is A° C., it is possible toknow which is the maximum value of the temperature experienced by thetemperature-history sensor, A° C. or higher or lower than A° C., fromthe measurement results of the temperature-history sensor. In the casewhere the temperature-history sensor has a plurality of settemperatures, it is possible to know which is the maximum value of thetemperature experienced by the temperature-history sensor, lower thanthe lowest set-temperature, higher than or equal to the lowestset-temperature and lower than the highest set-temperature, or higherthan or equal to the highest set-temperature. Furthermore, in the casewhere the maximum value is a temperature between the set temperatures,it is possible to know which set temperatures, among the plurality ofset temperatures, sandwich the maximum value. For example, in the casewhere the set temperatures are A° C., B° C., and C° C., the meltingpoints of the thermofusion materials are A° C., B° C., and C° C., and A°C.<B° C.<C° C. is satisfied, it is possible to obtain the informationregarding which is the temperature experienced by thetemperature-history sensor, lower than A° C., higher than or equal to A°C. and lower than B° C., higher than or equal to B° C. and lower than C°C., or higher than or equal to C° C.

The predetermined set temperature of the above-describedtemperature-history sensor is indicated on a display 17 included in thetemperature-history sensor. In the case where the temperature-historysensor has a plurality of set temperatures, all the set temperatures areindicated. A method for indicating the set temperature may be anindirect indication method, in which the set temperature is indicated bysome type of operation, or a method, in which the set temperature isdirectly indicated by, for example, a method by using printing, withoutthe need for operation. Examples of methods for indirectly indicatingthe set temperature include a method in which the information regardingthe set temperature is given to a memory device included in theabove-described temperature-history sensor, and the information is readby using an electrical method, a magnetic method, or an optical methodso as to indicate the set temperature. The case where the information,e.g., a type number, which indicates the type of tag, is given to thetag, and the above-described type number is checked against anotherinformation regarding the correlation between the type number and theset temperature so as to get the set temperature from the type number,is also included in the case where the information regarding the settemperature is given to the display. It is also possible that the taghas another resonance circuit, the resonance characteristic of theresonance circuit indicates the type number of the tag, and the settemperature is indicated by using the type number, as described above.Devices, e.g., bar codes and two-dimensional bar codes, in which paperis generally used as a base material, are included in the memorydevices. The method by using printing is a method in which the settemperature is printed by using letters or symbols in such a way thatthe set temperature can be visually checked. For example, in the casewhere the set temperatures of the temperature-history sensor are A° C.,B° C., and C° C., the set temperatures can be expressed as A−B−C. Thesedisplays may be disposed in the temperature-history sensors. In otherembodiments, the displays can be disposed separately from thetemperature-history sensor, insofar as it can be recognized that thedisplay indicates the set temperature of the above-describedtemperature-history sensor.

The display can indicate information other than the set temperature,besides the set temperature.

For the thermofusion materials 11 to 13, for example, petroleum wax andsynthetic wax, e.g., paraffin wax and polyolefin-based wax,thermoplastic resins, e.g., polystyrenes, methacrylic resins, andpolyethylenes, and natural fats and oils, e.g., animal fats and oils,can be used. Here, the thermofusion material refers to a material whichis fused due to heat so as to change from a solid state to a liquidstate.

The spacer 14 having a thickness suitable for forming a gap can be used,and resin films, adhesive films, and the like can be used therefor. Thethickness of the spacer is adjusted in accordance with the amount ofcapacitance required of the capacitor.

A method for forming a tag and a method for measurement will bedescribed below in detail with reference to specific examples.

A polyimide substrate is used as the insulating substrate 1, theelectroconductive wiring pattern 3, the pad portions, and the via holes15 are formed from copper on the above-described insulating substrate 1by a photolitho process and laser drilling. Film type hot melt adhesives(adhesion temperature 140° C. to 160° C.) are placed in parallel on thelower electrode 2 and are dried, so as to form spacers having a filmthickness of 75 μm. In this case, air is used as the dielectricmaterial, and the gaps between the spacers are used as-is. The capacitorupper electrode 9 separately formed by metal working is fixed to thelower electrode 2 with spacers therebetween so as to form a resonancetag. Regarding the resonance tag, a paraffin wax piece 120P having amelting point of 50° C. and serving as the thermofusion material 11, aparaffin wax piece 135P having a melting point of 60° C. and serving asthe thermofusion material 12, and a paraffin wax piece 155P having amelting point of 70° C. and serving as the thermofusion material 13 areset in the vicinity of three respective openings 10, so that atemperature-history sensor having set temperatures of 50° C., 60° C.,and 70° C. is prepared. The set temperatures are recorded as a bar code17 included in the temperature-history sensor, and the set temperaturescan be checked by indicating the information of the bar.

This temperature-history sensor is placed on a hot plate, and theresonance characteristic of the resonance tag included in thetemperature-history sensor is measured after changing the temperature.The resonance characteristic is measured after the tag and the hot plateheating surface reach the thermal equilibrium state sufficiently.Therefore, heating is performed up to a predetermined temperature, thetemperature is kept for 1 hour, and cooling is performed to roomtemperature. Thereafter, the measurement is performed.

FIG. 6 shows the measurement results of the resonance characteristic ofthe tag in the case where the temperature of the hot plate is set at 25°C. (room temperature, initial state), 52° C., 62° C., and 72° C. Theevaluation of the resonance characteristic of the tag can be performedby using a network analyzer (trade name HP8753E) produced byHewlett-Packard Company. Here, the resonance characteristic refers to aresonance frequency of a resonance peak of the tag or the amplitude andthe Q value attributed to the peak. Practically, the resonance frequencyis easily identified and, therefore, can be used for evaluating theresonance characteristic.

The resonance frequencies at the individual temperatures are 75.6 MHz at25° C., 65.5 MHz at 52° C., 58.5 MHz at 62° C., and 53.4 MHz at 72° C.The resonance frequency shifts to the lower-frequency side as theheating temperature increases. The resonance frequencies at theabove-described individual temperatures are taken as reference valuesand, thereby, an unknown temperature experienced by the tag can becalculated from the frequency of the tag. That is, when the resonancefrequency is 75.6 MHz, the temperature experienced by the tag isestimated to be lower than 50° C. When the resonance frequency is 65.5MHz, the temperature is estimated to be 50° C. or higher, and lower than60° C. When the resonance frequency is 58.5 MHz, the temperature isestimated to be 60° C. or higher, and lower than 70° C. When theresonance frequency is 53.4 MHz, the temperature is estimated to be 70°C. or higher. Such information of the reference values is given to thetag by a technique similar to that in the case where the informationregarding the set temperatures of the tag is directly or indirectlyindicated.

It can be observed that the paraffin wax pieces set in the vicinity ofthe opening portions of the upper electrode before heating have spreadinto the inside of the openings 10 after being heated to respectivetemperatures. It is believed that the paraffin wax serving as thethermofusion material reaches the melting point so as to fuse and flowinto the gap portion between the capacitor electrodes by a capillaryforce. Since the paraffin wax flows into the gap portion, the effectivedielectric constant increases, so that the resonance peak shifts to thelow-frequency side.

As described above, the fusion of the paraffin wax can also be checkedvisually. Therefore, the history of the temperature detected by theabove-described resonance frequency can easily be checked visually aswell. The principle is the inflow of the melt. Therefore, in the casewhere detection of the fact that solid-liquid transfer has occurred by afusion phenomenon due to heating, rather than detailed temperatureinformation, is good enough for the user, the fact can be easily knownby using the similar configuration through visual check and thefrequency shift. Since fine adjustments of the range of measurabletemperature history can also be made by, for example, mixing at leasttwo types of dielectric, the versatility is higher than that in the casewhere a temperature fuse, which goes into action merely at apredetermined temperature, is incorporated.

Second Embodiment

The present embodiment relates to a temperature-history sensor in whichat least a thermofusion material is included between capacitors, and thethermofusion material between capacitor electrodes flows out due to heatso as to change the effective dielectric constant of the capacitor.

FIGS. 7A and 7B and FIGS. 8A to 8D show a temperature-history sensor ofthe present embodiment. FIGS. 7A and 7B are plan views of the tag. FIG.7A shows a front-side view and FIG. 7B shows a back-side view. FIG. 8Ais a sectional view of a section cut with a VIIIA-VIIIA plane shown inFIG. 7A, FIG. 8B is a diagram of a shielding member 16 viewed fromabove, and FIG. 8C is a sectional view of a section of the tag cut witha VIIIC-VIIIC plane shown in FIG. 7A.

The tag is formed as in the first embodiment, except that thethermofusion materials 11 to 13 are not disposed in the vicinity of theopenings but are disposed as the dielectric materials between thecapacitor electrodes and that a piece of normal paper (shielding member16) for absorbing the thermofusion material, which are disposed betweenthe electrodes and which flow out due to fusion, is disposed in thevicinity of the openings. Consequently, in the present embodiment, forexample, the material described as the thermofusion material in thefirst embodiment can be used for the dielectric material. In FIG. 8C, apiece of paper is disposed on the upper electrode 9. However, the pieceof paper, or other such material, may be disposed at a position otherthan the above-described position insofar as the piece of paper can bedisposed in the vicinity of the opening portions. For example, as shownin FIG. 8D, the normal paper may be disposed in the flow direction ofthe thermofusion material due to fusion.

The temperature is changed in a manner similar to that in the firstembodiment, and the resonance characteristic of the resonance tag ateach temperature is measured. The thermofusion material, which isdisposed between the capacitor electrodes and which is fused due tochange of temperature, flows out, and is absorbed by the piece of paperdisposed in the vicinity of the opening portion. Consequently, theresonance characteristic of the resonance tag included in thetemperature-history sensor is changed.

FIG. 9 shows the measurement results of the resonance characteristic ofthe tag in the case where the temperature of the hot plate is set at 25°C. (room temperature, initial state), 52° C., 62° C., and 72° C.

In contrast to the first embodiment, the resonance frequency shifts tothe higher-frequency side as a higher temperature is experienced by thetag. The reason for this is believed to be that a larger amount of thethermofusion material flows out as the temperature increases, so as todecrease the effective dielectric constant of the capacitor.

Third Embodiment

In the third embodiment, a resonance tag as shown in FIGS. 10A and 10Band FIG. 11 is formed. FIGS. 10A and 10B are plan views of the tag. FIG.10A shows a front-side view and FIG. 10B shows a back-side view. FIG. 11is a sectional view of a section cut with a XI-XI plane shown in FIG.10A. A polyimide substrate is used as the insulating substrate 1, acopper pattern is used as the electroconductive wiring pattern 3, and aporous titanium oxide film (film thickness 10 μm) formed from titaniumoxide paste is used as the dielectric material 8. As in the firstembodiment, the electroconductive wiring pattern 3, the lower electrode2, the pad portion 4, the dielectric film 8, the upper electrode 9having the opening portions 10 are formed on the insulating substrate 1.Pad portions 5 to 7 are formed on the insulating substrate 1. The porousdielectric film 8 is also formed on the lower electrode. Thethermofusion materials 11 to 13 are disposed in the opening portions 10.

The porous dielectric film, which is the dielectric material 8, isformed as described below. Titanium oxide is put into water and nitricacid is added thereto. The resulting mixed solution is treated with aplanetary mill so as to prepare a dispersion of titanium oxide. In orderto improve variations in coating, polyethylene glycol (PEG, molecularweight 20,000) is added to the dispersion of titanium oxide, so that atitanium oxide paste is prepared. The resulting titanium oxide paste isapplied to the lower electrode 2 by a doctor blade method, heating anddrying is performed at 250° C. in an electric furnace, so as to producea titanium oxide film serving as the porous dielectric film. The filmthickness of the titanium oxide film measured with a probe filmthickness gauge at this time is 10 μm. In the present invention, theporous dielectric refers to a dielectric having many pores andexhibiting the porosity of 30% or more. Examples of such a porousdielectric include zirconia oxide, aluminum oxide, and barium titanate,besides the above-described titanium oxide. A resonance tag having adesired resonance frequency can be produced by using a porous dielectricfilm having the dielectric constant suitable for the desired resonancefrequency as the dielectric material 8. The upper electrode 9 of thecapacitor is formed by screen printing a silver paste on a patternhaving the opening portions 10 and, thereafter, performing a heattreatment at 200° C. Subsequently, paraffin wax pieces 120P, 135P, and155P having the melting points of 50° C., 60° C., and 70° C.,respectively, are set as the thermofusion materials in three respectiveopening portions 10, and are fixed with a Kapton® tape (not shown in thedrawing) from above, so as to produce a resonance tag. As in the firstembodiment, the set temperatures of the present embodiment are 50° C.,60° C., and 70° C., and as shown in FIG. 10A, the set temperatures areindicated as 50, 60, and 70 on a set temperature printing portionincluded in the temperature-history sensor.

This resonance tag is placed on a hot plate, and the resonancecharacteristic of the resonance tag at each temperature is measuredafter sequentially changing the temperature to 52° C., 62° C., and 72°C. The resonance characteristic is measured after the tag and the hotplate heating surface reach the thermal equilibrium state sufficiently.Therefore, heating is performed up to a predetermined temperature, thetemperature is kept for 1 hour, and cooling is performed to roomtemperature. Thereafter, the measurement is performed.

FIG. 12 shows the measurement results of the resonance characteristicwhere the temperature is changed. The resonance frequency result is 15.8MHz at room temperature, 25° C., in the initial state, 14.6 MHz at 52°C., 13.6 MHz at 62° C., and 12.8 MHz at 72° C. Therefore, as in thefirst embodiment, it is observed that the resonance frequency of the tagexperienced a higher-temperature environment shifts to thelower-frequency side. It can be observed that all of three paraffin waxpieces set in the opening portions 10 of the upper electrode 9 beforeheating have spread due to fusion. As is clear from this, when theheating temperature reaches the melting point of each paraffin wax, theparaffin wax is heat-fused so as to flow into the porous titanium oxidefilm from the opening portion. Since the paraffin wax flows into theporous titanium oxide film, the effective dielectric constant increases,so that the resonance frequency shifts to the low-frequency side.

As in the first embodiment, it is possible to measure an unknowntemperature, which has been experienced by the tag, by using thesemeasurement values as reference values.

For the dielectric film, porous titanium oxide film subjected to alipophilicity-imparting surface treatment may be used as the dielectricfilm. The heat-fused paraffin can flow into the porous titanium oxidefilm more speedily by subjecting the porous titanium oxide film to thelipophilicity-imparting treatment. Examples of suchlipophilicity-imparting treatments include a method in which a silanecoupling agent solution is dropped to the opening portion of the tagincluding the porous titanium oxide film, washing is performed withwater, and heating is performed at 110° C. to remove water, so that thelipophilicity-imparting treatment of the porous titanium oxide film isperformed.

Fourth Embodiment

A resonance tag is produced as in the third embodiment, except that apolystyrene film is used as the dielectric material 8 and for the upperelectrode, copper foil is cut and stuck on the polystyrene film.

The polystyrene film is formed by applying polystyrene fine particlessynthesized from a styrene monomer by a suspension polymerization methodand performing heat-drying at 70° C. It is checked by measurement with aprobe film thickness gauge that the film thickness of the porouspolystyrene film is 10 μm. The capacitor upper electrode 9 is producedby sticking copper foil, which has been cut with a pattern havingopening portions, on the polystyrene film. Paraffin wax pieces 120P,135P, and 155P having the melting points of 50° C., 60° C., and 70° C.,respectively, are set as the thermofusion materials in three respectiveopening portions, and are fixed with a Kapton® tape from above, so as toproduce a resonance tag. Thereafter, the measurement is performed underthe heat-measurement condition similar to that in the first embodiment.

FIG. 13 shows the measurement results of the resonance characteristic inthe case where the temperature of the hot plate is increased from roomtemperature, 25° C., to 52° C., 62° C., and 72° C. As a highertemperature is experienced, the resonance frequency shifts from 33.6 MHzat room temperature, 25° C., in the initial state to the lower-frequencyside, 31.0 MHz, 28.9 MHz, and 27.2 MHz. In the present embodiment, as inthe first embodiment, the resulting relationship between the temperatureand the resonance frequency is taken as the reference value and,thereby, it is possible to calculate an unknown temperature, which hasbeen experienced by the tag, from the resonance frequency. The resonancefrequencies are higher than those in the case where titanium oxide isused as the dielectric material. This is because the relative dielectricconstant of the porous polystyrene film is lower than that of titaniumoxide. In this manner, the peak wavelength of the resonancecharacteristic can be appropriately selected in accordance with thematerial to be used.

Fifth Embodiment

FIGS. 14A and 14B and FIGS. 15A and 15B show a tag of the presentembodiment.

FIGS. 14A and 14B are plan views of the tag. FIG. 14A is a front-sideview and FIG. 14B is a back-side view. FIG. 15A is a sectional view of asection cut with a XVA-XVA plane shown in FIG. 14A.

A resonance tag is produced as in the third embodiment, except thatnormal paper (shielding member) 16 printed with the set temperatures, asshown in FIG. 15B, is disposed on the upper electrode and on paraffinwax pieces as shown in the sectional view of FIG. 15A.

When a heating experiment similar to that in the third embodiment isperformed, as in the third embodiment, a melt flows into the porous filmas the temperature increases. Therefore, the shift of the resonancefrequency can be checked. The melt also flows into the normal papercovering the upper surface of the paraffin wax, and the manner ofsolidification can be observed after cooling is performed to roomtemperature. That is, it becomes possible to easily perform visual checkin combination with the temperature history detection based on thefrequency shift of the resonance peak by using a wireless system. Atthis time, each set temperature may be printed in the vicinity of thethermofusion material having the melting point equal to the settemperature in such a way that an experienced temperature can berecognized. In such a case, for example, as shown in FIG. 14A, thenumber “70” is inscribed in the vicinity of the thermofusion materialhaving the melting point of 70° C. In the case where the informationregarding the thermofusion materials and their melting points isseparately provided, the temperature history can be visually recognizedby checking the visual information against the above-describedinformation without printing the melting point in the vicinity of thethermofusion material.

In the present embodiment, as in the first embodiment, the resultingrelationship between the temperature and the resonance frequency istaken as the reference value and, thereby, it is possible to calculatean unknown temperature, which has been experienced by the tag, from theresonance frequency. The visual recognition is facilitated by usingcolored paraffin wax, which is prepared in advance by dissolving ordispersing fat dye or pigment into the paraffin wax to be used.

Sixth Embodiment

A temperature-history sensor is formed in a manner similar to that inthe third embodiment, except that a reservoir for stocking the fusedmaterial and a flow path are formed. FIGS. 16A and 16B are plan views ofthe tag. FIG. 16A is a front-side view. FIG. 17 is a sectional view of asection cut with a XVII-XVII plane shown in FIG. 16A. In the presentembodiment, the structure, in which the opening portions are disposed inthe electrode and the thermofusion materials are disposed therein, isnot adopted in contrast to the third embodiment. Instead, thethermofusion material 11 is disposed in a reservoir 18 of a flow path 19connected to the dielectric material 8. In particular, the reservoir 18is positioned at the end of the flow path 19. The flow path 19 and thereservoir 18 may be patterned in advance during the formation of acopper pattern. At this time, the portion, from which copper has beenremoved, becomes the flow path and the reservoir portion, and thethickness of the copper foil (here, thickness is 35 μm) corresponds tothe depth. A piece of paraffin wax 120P having a melting point of 50° C.is set as the thermofusion material 11 in the reservoir portion, and isfixed with a Kapton® tape 20 from above the thermofusion material 11 insuch a way that the flow path 19 and the reservoir 18 are also covered,so as to produce a resonance tag. The flow path and the reservoir may beformed by a method other than the method in which a copper pattern ispatterned in advance. For example, in a method referred to asnanoimprint, a mold having a fine uneven pattern is pressed against aresin and, thereby, a micro flow path can be formed. A resin providedwith such a groove may be used as an alternative to the flow path. Here,the explanation is made with reference to the example in which theporous film is used. However, the first embodiment, in which the gapportions are used, may be applied, as a matter of course.

This resonance tag is placed on a hot plate, and the resonancecharacteristic of the resonance tag is measured while the temperature ischanged. Heating is performed up to 50° C. and, thereafter the resonancecharacteristic with the passage of time is measured. Regarding the flowpath, a flow path A having a length of 1 cm and a flow path B having alength of 2 cm are prepared. The results are shown in Table 1.

When the heating is performed at 50° C., the resonance frequency staysat the initial stage of 15.3 MHz. However, the resonance frequencyshifts to 12.9 MHz, that is, the low-frequency side, after 30 secondshave elapsed in the case where the flow path A is used, or after 50seconds have elapsed in the case where the flow path B is used. Thisdepends on the time elapsed before the fused paraffin comes into contactwith the porous titanium oxide thin film portion. Since a capacitancechange occurs only after the tag has experienced a specific temperaturefor a specific time, the time elapsed before the frequency change can beadjusted by the length or the depth of the flow path. Therefore, theinformation regarding the time, whether the tag has experienced thespecific temperature for at least the specific time or not, can beobtained by forming a flow path for supplying the above-describedthermofusion material between the capacitor electrodes, in addition tothe information, whether a specific temperature has been experienced ornot, as in the case where a simple thermal-fuse-switch or the like isused.

TABLE 1 Time/sec Flow path A Flow path B  0 15.3 15.3 10 15.3 15.3 2015.3 15.3 30 12.9 15.3 40 12.9 15.3 50 12.9 12.9 60 12.9 12.9

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest reasonableinterpretation so as to encompass all modifications, equivalentstructures and functions.

This application claims the benefit of Japanese Application No.2006-329620 filed Dec. 6, 2006, which is hereby incorporated byreference herein in its entirety.

1. A temperature-history sensor comprising: a resonance circuitcomprising: a capacitor; and a coil; a display for indicating apredetermined set temperature of the temperature-history sensor; and athermofusion material between electrodes of the capacitor, wherein themelting point of the thermofusion material is in the region of the settemperature.
 2. The temperature-history sensor according to claim 1,wherein the region of the set temperature is the range from atemperature 0.5° C. lower than the set temperature to the settemperature.
 3. The temperature-history sensor according to claim 1,wherein the display indicates the set temperature by one of thefollowing methods: an electrical method; a magnetic method; an opticalmethod; and a method by using printing.
 4. The temperature-historysensor according to claim 1, wherein at least a part of the thermofusionmaterial between the capacitor electrodes flows out due to heat so as tochange the resonance characteristic of the resonance circuit.
 5. Atemperature-history sensor comprising: a resonance circuit comprising: acapacitor; and a coil; a display for indicating a predetermined settemperature of the temperature-history sensor; and a thermofusionmaterial, wherein the melting point of the thermofusion material is inthe region of the set temperature, and at least a part of thethermofusion material fuses due to heat and flows between electrodes ofthe capacitor so as to change the effective dielectric constant of thecapacitor.
 6. The temperature-history sensor according to claim 5,wherein the capacitor comprises a porous dielectric between thecapacitor electrodes.
 7. The temperature-history sensor according toclaim 5, wherein the thermofusion material is in the vicinity of thecapacitor electrodes.
 8. The temperature-history sensor according toclaim 5, further comprising a flow path for supplying the thermofusionmaterial to a space between the capacitor electrodes.